1. Field of the Invention
The invention relates in general to an apparatus and a method for generating chopper-stabilized signals, and more particularly to an apparatus and a method for generating chopper-stabilized signals in a thin-film transistor (TFT) liquid crystal display (LCD) without the use of a start control signal (STV).
2. Description of the Related Art
A chopper-stabilized signal is adopted to improve the problem of the offset voltage in an OP amplifier and also to enhance the uniformity.
FIG. 1 (Prior Art) is a schematic illustration showing a conventional OP amplifier 100. As shown in FIG. 1, the OP amplifier 100 has an output voltage VOA equal to an input voltage VI plug an offset voltage Vos(i) in a first chopper-stabilized mode chopper-A, and has an output voltage VOB equal to an input voltage VI minus the offset voltage Vos(i) in a second chopper-stabilized mode chopper-B. The OP amplifier 100 is alternately switched between the first chopper-stabilized mode chopper-A and the second chopper-stabilized mode chopper-B. Consequently, the OP amplifier 100 has an average output voltage VAVG equal to the input voltage VI, and the problem of the offset voltage is thus solved.
The method of solving the offset voltage of the OP amplifier according to the chopper-stabilized signal is also applied to a thin-film transistor (TFT) liquid crystal display (LCD) frequently. FIG. 2 (Prior Art) is a simple schematic illustration showing a conventional TFT LCD 200. Referring to FIG. 2, the TFT LCD 200 has many rows of pixels. In FIG. 2, 1072 rows of pixels and four frame time periods are illustrated as an example. Usually, in the application field of the TFT LCD, it is requested that the OP amplifier in each row of pixels has to be kept in the first chopper-stabilized mode chopper-A and the second chopper-stabilized mode chopper-B for the same period of time in at least every four frame time periods. For the purpose of the clear representation, the first chopper-stabilized mode chopper-A is represented by A, and the second chopper-stabilized mode chopper-B is represented by B.
In the first frame time period, the OP amplifiers in all the rows of pixels of the TFT LCD 200 are kept in the first chopper-stabilized mode chopper-A. Then, in the second frame time period, the OP amplifiers in all the rows of pixels are kept in the second chopper-stabilized mode chopper-B. The modes of the OP amplifiers are alternately switched in this manner. After the fourth frame time period, the OP amplifier in each row of pixels is kept in the first chopper-stabilized mode chopper-A and the second chopper-stabilized mode chopper-B for the same period of time, so the problem of the offset voltage is solved and the uniformity is enhanced.
However, the conventional method of switching between the first chopper-stabilized mode chopper-A and the second chopper-stabilized mode chopper-B in the TFT LCD 200 is to distinguish whether the frame time period is changed and thus to perform the switching operation according to a start control signal (STV) outputted from a timing controller (not shown) of the TFT LCD 200 at the beginning of each frame time period. However, some current TFT LCDs do not provide the start control signal. So, the method of solving the offset voltage of the OP amplifier is not completely suitable for all the TFT LCDs.